1. Field of the Invention
The invention relates to frequency compensation circuit, and in particular relates to frequency acquisition and tracking circuit.
2. Description of the Related Art
OFDM (Orthogonal Frequency Division Multiplexing) is a very popular data transmission technique in recent years. OFDM technique is to divide an OFDM signal onto multiple sub-carriers that can be transmitted simultaneously. Although the sub-carriers are overlapped, one sub-carrier does not interfere with another sub-carrier due to their orthogonality.
The OFDM signal is sensitive to synchronization error, due to the narrow interval between sub-carriers. This synchronization error comprises a CFO (Carrier Frequency Offset) and an SFO (Sampling Frequency Offset). CFO is caused by the difference is the oscillation frequency, the Doppler effect and the phase noise between a transmitter and a receiver, and SFO is caused by the difference of the sampling frequency between the transmitter and the receiver.
FIG. 1 illustrates a conventional phase compensation circuit comprising frequency acquisition circuit 12, FFT (Fast Fourier Transform) circuit 14 and frequency tracking circuit 16. Frequency tracking circuit 16 comprises multiplier 162, phase error detector 164, loop filter 166 and NCO (numerical controlled oscillator) 168.
FIG. 2 is a schematic view showing a conventional frequency acquisition circuit 12. Frequency acquisition circuit 12 comprises multiplier 202, correlator 204, phase evaluator 206, phase accumulator 208 and NCO (numerical controlled oscillator) 210.
In frequency acquisition circuit 12, correlator 204 generates a conjugate value according to two symbols with the same preamble. A first phase angle signal is generated by phase evaluator 206 according to the conjugate value and the symbol length, wherein the first phase angle signal is calculated by a CORDIC circuit. A first accumulated phase angle is generated by phase accumulator 208 by the way of accumulating the first phase angle signals of different symbols. A first phase compensation signal is generated by NCO 210 through searching the lookup table according to the first accumulated phase angle. An initial compensation signal is generated by multiplier 202 by compensating a received signal according to the first phase compensation signal.
Then, FFT circuit 14 performs the fast Fourier transformation on the initial compensation signal and generates a first signal and the first signal is transferred to the frequency tracking circuit 16.
In frequency tracking circuit 16, a second phase angle signal is generated by phase error detector 164 according to the first signal, wherein the second phase angle signal is calculated by the CORDIC circuit. The second phase angle signal is the phase difference between two adjacent symbols. The equation is as follows:
            e      f        ⁡          (      n      )        =                    ∑        k            ⁢              phase        ⁡                  (                      X                          n              ,              k                                )                      -                  ∑        k            ⁢              phase        ⁡                  (                      X                                          n                -                1                            ,              k                                )                    
wherein n is the index of the symbol, k is the index of the pilot in the symbol, phase is a function used to calculate the phase of the pilot. Then, the high frequency part of the second phase angle signal is filtered by loop filter 166. The transfer function of loop filter 166 can be
            F      ⁡              (        z        )              =                  C        1            +                        C          2                          1          -                      Z                          -              1                                            ,wherein two parameters C1 and C2 can be used to determine the bandwidth and the convergence rate of frequency tracking circuit 16. NCO 168 is composed of a phase accumulator and a sin/cosine lookup table, wherein a second accumulated phase angle is generated by the phase accumulator by accumulating the second phase angle signals of different filtered symbols and a second phase compensation signal is generated by looking up the lookup table according to the second accumulated phase angle. The transfer function of NCO 168 can be
            N      ⁡              (        z        )              =                            K          0                ⁢                  z                      -            1                                      1        -                  z                      -            1                                ,wherein K0 is constant gain. Finally, an output signal Output is generated by multiplier 162 through compensating the first signal according to the second phase compensation signal.
Frequency acquisition circuit 12 and phase error detector 164 of frequency tracking circuit 16 is the core of the phase compensation circuit. However, the performance may be affected by the phase evaluation algorithm used in conventional phase compensation circuit. The high hardware complexity of the CORDIC circuit used to calculate the phase angle may increase the hardware cost and may further increase the power consumption. Therefore, an algorithm with low hardware complexity is desired to calculate the phase angle to lower the power consumption of the circuit.